Canikon MUST switch to EVFs

All other things being equal (which they aren't on different lens designs), a shorter registration distance doesn't help when the focal length is greater than the registration distance.

Have a look:

14mm lens for a 44mm registration distance.  Note the rear element is all the way at the back.  A shorter registration distance would help to get that element even closer to the sensor:

135mm lens for a 44mm registration distance.  Note that the rear element is not all the way at the back.  Extra room is available.  A shorter registration distance wouldn't change this lens design one bit:

A standard zoom.  Again, the rear element is at the back and a shorter distance would help:

A telephoto zoom.  Like above, the rear element isn't even using up the room available, thus a shorter registration distance wouldn't change the design:

With a longer minimum focal length (the above is 70mm, this one is 100mm), the rear element moves even farther forward:

This effect gets more pronounced at long focal lengths.  400mm on a 44mm registration distance.  Note how far forward the rear element is.  Making the registration distance shorter is just going to make the empty back of the lens longer:

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Lee Jay
(see profile for equipment)